#include<bits/stdc++.h>
using namespace std;
int main(){
	double d,x,y,t;
	while(scanf("%lf %lf %lf %lf",&d,&x,&y,&t) != EOF){
		if(((d/x)+t) < (d/y))
			printf("YES\n");
		else
			printf("NO\n");
	}	
	return 0;
}